This invention pertains to the formation of gate structures for semiconductor devices, and particularly for microwave semiconductor devices. More particularly, this invention relates to the formation of gates of extremely small length.
The fabrication of metal gate electode semiconductor FETs has received increased attention in the semiconductor industry in recent years, especially in that segment regarding high frequency microwave operation. Of particular interest has been the formation of gate electrodes of particularly small length in the sub-micrometer range with low electrical resistance along the width of the gate. One particular method for forming such FET gates is described in U.S. Pat. 4,213,840, which is incorporated herein by reference.
In processes of this type, it is conventional to form a gate electrode that has, in vertical transverse cross-section perpendicular to the width of the electrode, a T or Y configuration. The narrow portion or stem of the electrode is that actually in contact with the substrate or semiconductor body, and overlying that stem is a broader portion of deposited metal, the broader portion being particularly useful in reducing resistance along the width of the gate electrode, as well as providing for ease of bonding thereto.
The particularly critical step in the formation of such electrodes is the photolithographic techniques practiced in establishing the photo-resist mask which will define the stem or substrate connection of the gate electrode. Conventionally, as in U.S. Pat. No. 4,213,840, this photolithographic definition is accomplished through the use of a positive image "dark field" mask. In this process, a photo-resist layer is first coated on the surface of the substrate which will bear the gate electrode. Thereafter, a mask is applied, with an opening or transparent portion which will define the area wherein the gate is to be formed, and the photo-resist is irradiated through the mask. That portion of the photo-resist underlying the opening in the mask which will define the gate electrode stem is affected by the radiation, frequently causing cross-linking of the photo-resist material, altering its solubility. The remainder of the photo-resist, shielded by the mask, is not so altered. After irradiation, the photo-resist is treated with a solvent, to either remove the irradiated and cross-linked material, or the non-irradiated material, depending on the operator's choice of processing techniques.
One phenomenon common to this type positive image or dark field mask photolithographic technique is the formation of an image substantially larger in cross-sectional dimension than the opening in the mask itself. This phenomenon is illustrated in FIG. 1. Photo-resist layer 102 lies deposited on the surface of substrate 100, on which is to be formed the gate electrode. Overlaying photo-resist 102 is mask 104, with opening 106, defining the theoretical gate formation area 108 of length X. However, as the surface of mask 104 and the photo-resist underlying opening 106 is irradiated, incident irradiation, combined with lateral cross-linking on light overexposure, causes a portion 110 of the photo-resist to be irradiated. As a result, when the mask is removed and the selected portion of the photo-resist 102 is developed, actual gate formation area 112 is left, which is substantially larger, in surface area, than theoretical and desired formation area 108. Thus, particularly when gate length at or substantially below 1 micrometer are desired to be created, it is particularly difficult to use positive image or dark field masking techniques, as the length of the gate stem formed is always substantially larger than the mask opening, and larger than desired. For example, a 0.8 micrometer mask opening might result in a 0.9 to 1.0 micrometer photo-resist opening.
Accordingly, it is an object of this invention to provide a process whereby gate electrodes of small length and low resistance along the gate width may be formed.
Another object of this invention is to provide a process whereby gate electrodes of gate stem length substantially below 1 micrometer can be formed.
Yet another object of this invention is to provide a process whereby gate electrodes whose stem length is always less than the length of the mask used to define that stem length may be formed.
These and other objects of this invention are attained by employing a negative image "light field" mask to define the area in which the gate electrode stem is to be formed on the surface of the semiconductor substrate support. A photo-resist is applied over the surface of the substrate on which the gate electrode is to be formed. Thereafter, a mask which is opaque to irradiation, over the area in which the gate electrode stem is to be formed is applied over the photo-resist, and the photo-resist subsequently irradiated. A portion of the photo-resist, which will always be smaller in dimension than the opaque portion of the mask will not be irradiated, while all other portions (the light field) will be irradiated, and their solubilities thus being altered. Thereafter, the irradiated photo-resist is removed by a development wash, leaving behind only the thin photo-resist line defining the stem of the gate electrode. A dielectric layer is deposited over the surface of the substrate and photo-resist line, and thereafter the photo-resist line is removed, along with the overlying dielectric forming an opening in the dielectric layer to the substrate surface smaller in dimension than the photo-resist mask. A layer of metal is deposited to form the stem of the gate electrode in the opening. Thereafter, a mask is set down to define a cap area overlapping the stem on either side, and the metal layer is removed, except in the defined area. Therafter, the remaining processing of the substrate is conducted including the removal of the mask.
Alternatively, the dielectric layer may be formed before the photo-resist is applied and a similar process is used to produce a metal mask to form the opening in the dielectric layer. Also, a second gate metal layer may be formed and used as a mask in forming the defined cap area.